Milestone 2: Implement execution core architecture

[Antisource]

Summary

Implements Milestone 2: Execution Core.

This PR introduces the foundational execution layer required for
running, observing, and summarizing autonomous system executions.

What changed

Execution lifecycle

• Added ExecutionContext as the shared execution boundary.
• Added Runtime abstraction for framework-independent execution.
• Added Executor orchestration layer.

Observability

• Added immutable Event model.
• Added Recorder for ordered execution history.
• Added lifecycle events:
  • execution.started
  • execution.completed
  • execution.failed

Execution outcomes

• Added ExecutionResult model to represent final execution state.

Architecture

The execution flow is now:

Experiment
    |
ExecutionContext
    |
Executor
    |
+-----------+
|           |
Runtime   Recorder
             |
           Events

Executor returns ExecutionResult.

Validation

Passed:

• ruff check
• ruff format
• mypy
• pytest

Next milestone

Milestone 3 will build trajectory and evaluation capabilities on top
of the execution core.